fix: add . to readme (#3538)

## What ❔

dummy change to trigger release-please PR

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## Checklist

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core/bin/external_node/README.md

@@ -6,7 +6,7 @@ Note: this README is under construction.
 
 ## Local development
 
-This section describes how to run the external node locally
+This section describes how to run the external node locally.
 
 ### Configuration